The goal of this project is to understand the steps involved in the control of testicular function. The inability of the fetal/immature Leydig cell to be desensitized by gonadotropin, a characteristic of the adult cell, is due to the absence of an estradio(E2)-mediated regulation of the androgen pathway (17 alpha-hydroxylase/17/20 demolase). In fetal rat Leydig cell cultures employed to analyze this differential response, E2 caused an up-regulation of its receptor and an induction of the regulatory mechanism of the androgen pathway. The absence of this endogenous regulation in fetal life is due to very low levels of aromatase activity, undetectable E2 production, and consequent low levels of E2 receptor. The above explains the low levels of E2-regulated 27K protein that is present in fetal cells and increased by E2 treatment. This long-lived protein is undetectable post-natally and increases with age, and is present in cytoplasmic matrix near the cisternae of RER and SER. Future research with this system will help to clarify mechanisms responsible for emergence of the adult cell population. These cultures were also used for studies on GnRH agonists, which cause a lesion of microsomal enzymes that markedly inhibits steroid production. GnRH receptors were undetectable in fetal testes, appeared post-natally and increased with age. In fetal cultures, GnRH receptors were up-regulated by GnRH and reduced by LH treatment. Thus, GnRH receptors via GnRH related peptides can influence the actions of LH on the fetal cell population. Opiate receptors and actions were present in Sertoli cells. Opiate up-regulated its receptor and reduced FSH-stimulated androgen binding protein. Opioids of Leydig cell origin may modulate Sertoli cell function and influence the transport of testosterone to germinal cells. Testicular converting enzyme in mature rats was mainly associated with germinal cells (spermatids), results consistent with hormonal studies. The Leydig cells contain low quantities of converting enzyme, 0.46 mIU/ng vs 29.1 mIU/mg in spermatids. Thus, tubular and locally produced angiotensin II could negatively modulate LH stimulation of Leydig cells and also modulate tubule function.